Eco-evolutionary dynamics of partially migratory metapopulations in spatially and seasonally varying environments

Predicting joint population dynamic and evolutionary responses to
environmental changes requires understanding interactions among
environmentally induced phenotypic variation, selection, demography and
genetic variation, and thereby predicting eco-evolutionary dynamics
emerging across diverse temporal and spatial scales. Partially migratory
metapopulations (PMMPs) feature seasonal coexistence of resident and
migrant individuals across multiple spatially distinct subpopulations.
Such PMMPs have clear potential for complex spatio-seasonal
eco-evolutionary dynamics because selection on migration impacts spatial
population dynamics, which in turn shape the form and magnitude of
selection, causing feedbacks with ongoing micro-evolution. However, the
key genetic and environmental conditions that maintain migratory
polymorphisms, and emerging eco-evolutionary dynamics of PMMPs under
stochastic environmental variation and strong seasonal perturbations, have
not yet been characterised. We present a general individual-based model
that tracks eco-evolutionary dynamics in PMMPs inhabiting spatially
structured, seasonally varying landscapes, with migration formulated as a
quantitative genetic threshold trait. Simulations show that such genetic
and landscape structures, which commonly occur in nature, can readily
produce a variety of stable partially migratory systems given diverse
regimes of spatio-seasonal environmental variation. Typically, partial
migration is maintained whenever sites differ in non-breeding season
suitability resulting from variation in density-dependence, causing ‘ideal
free’ non-breeding distributions where residents and migrants occur with
frequencies that generate similar survival probabilities. Yet, stable
partial migration can also arise without any fixed differences in
non-breeding season density-dependence among sites, and even without
density-dependence at all, through risk-spreading given sufficiently large
stochastic environmental fluctuations among sites and years. Further,
major local non-breeding season mortality events, such as could result
from extreme climatic events, can generate eco-evolutionary dynamics that
ripple out to affect breeding and non-breeding season space use of
subpopulations throughout the PMMP, on both short and longer timeframes.
Such effects result from spatially divergent selection on both the
occurrence and destination of migration. Our model thus shows how
facultative seasonal migration can act as a key mediator of
eco-evolutionary dynamics in (meta)populations occupying spatially and
seasonally structured environments, providing key steps towards predicting
responses of natural partially migratory populations to ongoing changes in
spatio-seasonal patterns of environmental variation.